In power plants where more than one boiler module is used to generat steam in separate steam generators for the respective boiler modules, steam from the respective steam generators may drive a steam turbine which is common to the boiler modules. If the steam generator in each boiler module comprises a high-pressure superheater and a reheater, the high-pressure steam from the different boiler modules is passed via valves to a common high-pressure turbine, where the steam expands and delivers energy, whereafter the steam is returned to the boiler modules for reheating in a reheater in the respective boiler module. The steam from the different reheaters is then passed to an intermediate-pressure and low-pressure turbine common to the modules, whereafter the steam, after condensing, is passed to a tank for feedwater, from which the water is utilized for new steam generation.
One reason for utilizing fewer turbines than the number of boiler modules is the considerable saving of costs for the installation of fewer steam turbine units in the plant.
In the normal operating range, that is, 30% to 100% of the power output from the plant, sliding pressure control is currently used. Sliding pressure control means that the pressure of the high-pressure steam, (live steam) and the pressure of the reheater steam are approximately proportional to the steam flow. During start-up and shutdown of the plant and in case of turbine trip (emergency stop of the turbine), the control of the boiler and the turbine, is performed in the most careful way. This means that the pressure in the reheater, determined by the pressure in the intermediate-pressure and low-pressure turbine, and hence the volume flow (the steam rate) through the high-pressure superheater and the flow volume through reheater in the load range, vary within acceptable limits without any special control.
If two boilers comprising reheaters are installed with a common steam turbine for both boilers, no problems arise for the high-pressure outlet of the steam turbine and for the reheaters of the boilers since the two boilers are driven with the same power. The individual boilers and the steam turbine have the same load expressed as a percentage. On the other hand, the distribution of the steam flows of the reheaters in the two boilers must be controlled in relation to each other with the aid of valves.
Control difficulties for a plant concept with two boilers and a common steam turbine arise when the two boilers are driven with different loads. The steam pressure before the intermediate-pressure turbine will then be the sum of the current reheater steam flows of the respective boilers divided by the design flow of the two reheaters. Before the high-pressure turbine the same conditions as with the intermediate-pressure turbine apply for the steam pressure. In other word, the sum of the steam flows in the two superheaters for high-pressure steam in the respective boilers divided by the design steam flows of these superheaters gives the pressure before the high-pressure turbine. If, for example, one boiler is driven with 50% load and the other boiler is driven with 100% load, the steam turbine will be driven with 75% of nominal full load. This means that the steam turbine receives an inlet steam pressure which amounts to 75% of the full load pressure. The superheater for live steam in the boiler which is driven with 100% load is designed so as to have, at this load, a volume flow and a steam rate which require full pressure in the superheater. Because of this, it is required that a higher pressure is applied to the superheater in this full-load boiler than what is justified by the inlet pressure at the steam turbine. In this connection, a throttle valve, for example, is utilized at the boiler outlet of the super-heater, whereby the pressure of this superheater is raised. A disadvantage of using a throttle valve is that an extra throttling loss is obtained.
On the other hand, the superheater in the boiler which is driven with only 50% load is subjected to a pressure which is 50% too high. This provides a considerably lower volume flow of steam through the superheater compared with the case where the boilers are working under equivalent conditions.
The steam pressure at the intermediate-pressure turbine, which is determined by the flow through the intermediate-pressure and low-pressure turbine, will also lie at 75% of the full-load pressure. The steam flow in the reheaters for the respective boiler must be distributed such that the reheater of each boiler is supplied with the correct steam flow in relation to the live steam flow out of the corresponding boiler. The steam pressure of the reheaters will be incorrect for both boilers in the same way as in the case of the above-described unbalance between the live steam pressures in the respective boilers. The pressure at the outlet of the high-pressure turbine, which is common to both boilers, must be maintained to avoid too high steam rates in the reheater belonging to the full-load boiler. As a result, the expansion line for the high-pressure turbine is reduced. Therefore, the output power from the plant is reduced compared with the output power which is achieved during parallel running of the two boilers with the same load, where the sum load for both boilers is the same as in the example described. Because of the reduced expansion, the temperature at the outlet from the high-pressure turbine is at the same time higher than the calculated temperature in the heat transfer area for the reheater. The temperature has to be reduced by, for example, water injection, which results in deteriorated efficiency. The boiler which is driven at only 50% of full load is, on the other hand, exposed to the opposite conditions. The reheater in this boiler senses too high a pressure which causes too low a volume flow whereas the temperature of the inlet steam is too low, which in turn contributes to the temperature of the outlet steam from the reheater becoming too low. These circumstances create demands for additional water injection into the reheater in this partial-load boiler, whereby a high pressure drop across the reheater is accomplished. Also, these circumstances contributes to a deterioration of the efficiency of the plant when the plant is not driven with equivalent loads in the two boilers. The lower temperature of the steam out of the reheater related to the steam turbine load may cause a problem for the steam turbine according to ruling standards.